1. Field of the Invention
This invention relates to an electrically heated automobile glazing comprising an electrically conductive transparent surface coating serving as a resistance heater, current supply conductors deposited along two opposite edges of the glazing and a decorative frame (frame-forming layer) of an opaque enamel, in particular a bakable enamel.
2. Background of the Related Art
Heated automobile glazings in which the resistance heater is a transparent surface layer are known. Since the transparent heating layer, which as a general rule is a multilayer system having a metallic conductive layer, is advantageously to be protected from mechanical and atmospheric influences, as a general rule it is deposited within a laminated glazing. For example, it can be deposited in a particularly advantageous way directly on the surface of one of the two individual glass sheets forming a laminated glazing, which is connected to the other individual glass sheet by an insert layer of a transparent plastic such as polyvinyl butyral.
When the glass sheets are mounted in the automobile body by a gluing process, they are provided, in addition, with a frame-forming layer of an opaque enamel. This frame-forming layer has as its object to prevent an external view of the adhesive layer. At the same time, this opaque frame protects the adhesive layer from UV rays.
In a known process for the production of such automobile glazings of laminated glass, the frame-forming marginal strip of an opaque bakable enamel is first deposited and dried on the inside surface of the outer glass sheet of the laminate. Then in a second printing operation, an electrically conductive printing paste made of a bakable enamel containing metallic silver is applied to form the current supply bars. After baking the two enamels, the transparent conductive layer is applied, for example by cathode sputtering in a magnetic field, on the glass sheet carrying the opaque frame and current supply bars. Processes of this type are described in detail, for example, in U.S. Pat. Nos. 4,744,844 and 4,654,067.
The known art also comprises the application of the electrically conductive surface coating as a first layer on the glass sheet of the laminate closest to the passenger compartment, on the face of the latter which faces the outside glass sheet, and the application of the current supply bars in the form of strips of a conductive silver paste suitable for printing on the surface coating. The frame-forming opaque marginal strip should, in this embodiment, be deposited on the inside of the outside glass sheet, because it must prevent the viewing of the current supply bars.
The known art also includes conductive black bakable enamels as well as the use of such electrically conductive bakable enamels for printing the strip forming a frame, called a decorative frame, parts of this decorative frame being used directly as heating conductors (U.S. Pat. No. 4,910,380). In this way, the lower part of the decorative frame located in the rest area of the windshield wipers can, for example, serve for heating this area.
For all known automobile glazings with surface heating of the type specified, not only is a double printing operation necessary for application of the decorative frame and for that of the current supply bars but also the two layers applied by printing must be subjected to a drying process in separate operations. The current supply bars applied by printing further have the disadvantageous property that, because of the thinness of the printing layer, the conductive cross section is relatively small, so that in case of high heating energy, the current supply bars are excessively heated.